1. Field of the Invention
The present invention relates generally to a white balance control circuit, and is directed more particularly to a white balance control system suitable for use in a color image pick-up apparatus.
2. Description of the Prior Art
A prior art white balance control circuit will be described with reference to FIG. 1, in which three CCD's (charge coupled device) are used as imaging elements or imagers for developing primary color signals or red, green and blue primary color signals R, G and B. In this case, three CCDs 1R, 1G and 1B are arranged in the horizontal direction with a shift of 1/3x (x being the arranging pitch of picture elements in the horizontal direction) between adjacent CCDs successively, as shown in FIG. 2. That is, an image pick-up device formed of three CCDs 1R, 1G and 1B arranged above is used to pick up an object (not shown) through red, green and blue color filters, respectively.
FIG. 3 is the graph which shows the relation between the response of the three primary color signals R, G and B and the phase thereof. In the graph of FIG. 3, the solid line represents the fundamental band component, the one-dot-chain line represents the side band component, and .tau..sub.H designates the sampling period of each picture element and corresponds to the time in which the horizontal scanning moves across the arranging pitch or distance x between adjacent picture elements.
Turning back to FIG. 1, picked up outputs R, G and B from the imager elements 1R, 1G and 1B are applied through sampling hold circuits 2R, 2G, 2B and pre-amplifiers 3R, 3G and 3B to process-amplifiers 4R, 4G, 4B, respectively, and the outputs therefrom are fed to an encoder 10 as may be well known.
In the encoder 10, a matrix circuit 12 is supplied with the outputs from the pre-amplifiers 4R, 4G, 4B and then produces a luminance signal Y and color difference signals R-Y and B-Y. The luminance signal Y therefrom is applied through a low pass filter 14Y to a composite or adding circuit 15, while the color difference signals R-Y and B-Y are applied through low pass filters 14R and 14B, which pass therethrough the frequency band components up to about 1.0 MH.sub.z, respectively, and clamp circuits 16R and 16B, which serve to reproduce DC components, respectively, to modulators 17R and 17B, respectively. The modulated output signals therefrom or carrier chrominance signals are applied to the adding circuit 15 to be added to the luminance signal Y. Thus, the adding circuit 15 produces a composite color television signal and delivers the same to an output terminal 15a. Reference numeral 20 designates a white balance control circuit which generally controls the red and blue primary color signals R and B based upon the green primary color signal G as the reference to achieve the white balance control. To this end, gain control circuits 21R and 21B are provided in the signal transmission paths of the red and blue primary color signals R and B, respectively.
The respective primary color signals R, G and B, which are fed to the process-amplifiers 4R, 4G and 4B, are also fed to a pair of subtracting circuits 22R and 22B in the white balance control circuit 20 from which primary color difference signals R-G and B-G are delivered. The primary color difference signals R-G and B-G are fed through low pass filters 23R and 23B, which act to remove undesirable side band components, to clamp circuits 24R and 24B for reproducing DC components. The clamped outputs therefrom are respectively fed to level comparing circuits 25R and 25B to be level-compared with a reference level from a reference potential source E whose potential is selected as the clamp level in this example.
The compared outputs from the comparing circuits 25R and 25B are respectively fed to detecting circuits 26R and 26B to be detected and the detected outputs therefrom are respectively fed through integrating circuits 27R and 27B to memory circuits 28R and 28B whose outputs are applied to the gain control circuits 21R and 21B as their gain control signals, respectively.
In FIG. 1, SWR and SWB designate switches which are connected between the integrating circuits 27R, 27B and the memory circuits 28R, 28B and used upon controlling or adjusting the white balance, respectively.
When the white balance is controlled or adjusted, if a white board is picked up and the switches SWR and SWB are made ON, the gain control circuits 21R and 21B are so controlled automatically that the levels of the primary color signals R and B become the same as the level of the primary color signal G which is the reference. After the white balance is controlled or adjusted, the switches SWR and SWB are made OFF. After the white balance is thus controlled or adjusted, the gain control is carried out by the outputs from the memory circuits 28R and 28B.
In the case that an image pickup tube such as a vidicon tube is used in place of a semiconductor element, for example, a CCD as the imaging elements 1R, 1G and 1B, the low pass filters 23R and 23B, which are provided for removing the undesirable side band components in the example of FIG. 1, become unnecessary.
As described above, according to the prior art white balance control circuit, the white balance is controlled or adjusted in accordance with the respective primary color signals R, G and B which are fed to the process-amplifiers 4R, 4G and 4B. When the white balance is perfect, the carrier chrominance signals must be balanced. In fact, however, there may occur such a phenomenon that R.noteq.G and B.noteq.G appear in the carrier chrominance signals by the influence of the signal transmission system from the process-amplifiers 4R, 4G and 4B to the modulators 17R and 17B. For example, due to the scattering of the circuit elements or secular changes thereof, the balance is disturbed.
Therefore, it is difficult in the above prior art that the white balance is controlled or adjusted perfectly and also the white balance control circuit 20 itself of the prior art becomes complicated in the circuit construction.